1. Field of the Invention
The present invention relates to a process for producing rods or blocks of semiconductor material which expands on solidification by crystallization of a melt produced from granular material, and also apparatus for carrying out the process.
2. The Prior Art
Mono- or polycrystalline rods or blocks of silicon are usually produced by the Czochralski crucible-drawing process, by zone drawing without a crucible or by casting processes in which a silicon melt is cast into molds of quartz or graphite and is subjected there to controlled solidification. Zone drawing requires polycrystalline stock rods produced by gentle gas phase deposition under carefully monitored conditions. Although the starting material used in crucible drawing or casting processes can be pieces of polycrystalline silicon deposited with less effort in comparison, the crucibles or molds used are a possible cause both of contamination and of additional costs. Similar problems also occur, because of the long contact time of molten silicon with vessel walls of foreign material, for example, in the process according to German Patent 2,925,679 or in the corresponding U.S. Pat. No. 4,312,700, in which a silicon melt produced in a separate melting crucible is fed into a crystallization chamber in which there is maintained a pool of melt from which the crystallizing material is drawn off downwards. The same applies to the process according to German Patent 3,531,610, in which the pool of melt is enclosed by rollers manufactured from material inert to silicon, the molten silicon being poured into the free surface thereof, while the solidifying material is drawn off downwards in a similar way to the previously mentioned process. While the casting processes mentioned are essentially used for the production of base material for solar cells, the rods obtained by crucible or zone drawing are usually sawn into wafers from which, in most cases, electronic or power components are then manufactured. The fields of application of the individual process are limited; thus, for example, casting or crucible-drawing processes do not allow for the production of material which corresponds to zone-drawn material, for example, with respect to the type and proportion of impurities. In addition, the energy balance of some processes is unfavorable: while on the one hand, a relatively large amount of melt has to be prepared with high energy consumption, after crystallization a similar significant amount of energy has to be conducted away again by cooling.